Hydrocarbon mixtures boiling in the kerosine range can be applied as fuel for aviation turbines; hydrocarbon mixtures boiling in the gas oil range can be used as a source of diesel engine fuel. It is important that the heating of such fuels does not cause the fuel to degrade and form tenacious deposits within the fuel system as this would ultimately lead to reduction of the life time of components such as oil-coolers, fuel control units and injector feed nozzles. Therefore, in order to be applicable in engines, the kerosine and gas oil fractions must have a very low metal content, especially a low copper and/or iron content, as (these) metals are considered to catalyze such deposition. Another compound which should not be present in fuels is water, as it can form acids with some impurities such as sulphur compounds. This causes rust and corrosion of tanks, lines and engines. Further, the presence of water in kerosine can lead to ice-formation.
It is known to use membranes, such as polysiloxane-type membranes, for separating various hydrocarbon compounds from each other, which compounds are present in considerable quantities and/or which compounds differ greatly from each other in molecular weight, chemical and/or physical properties. Such application is e.g. described in U.S. Pat. No. 4,810,366. From this patent it is known to separate hydrocarbon fluids such as lubricating base oils, from solvents using a hydrocarbon oil-selective membrane. The hydrocarbon fluids may include kerosine and gas oil. Suitable solvents include furfural and toluene.
In German Patent Application No. 2,741,486 a membrane is disclosed for the separation of water from hydrocarbons such as kerosine. The membrane comprises a random copolymer of a first monomer consisting of acrylic acid and/or methacrylic acid, and of a second monomer selected from the group consisting of acrylic acid, methacrylic acid, maleic acid and maleic anhydride.
Surprisingly, it has now been found that low concentration of metal compounds in gas oil and kerosine fractions, can be lowered further with the help of metal-selective membranes. Furthermore, it has been found that the permeate fractions obtained had a lower water content.